The word polyp refers to any overgrowth of tissue from the surface of mucous membranes. Intestinal polyps grow out of the lining of the small and large bowels. Polyps come in a variety of shapes—round, droplet, and irregular being the most common.
Polyps are one of many forms of tissue overproduction that can occur in the body. Cells in many body tissues sometimes keep growing beyond their usual limits. Medical scientists call this process neoplasia, which means simply "new growth." An individual overgrowth is called a neoplasm. In most cases these growths are limited, and the result is a benign swelling or mass of cells called a tumor. If the new growth occurs on the surface of the tissue instead of inside an organ it is often called a polyp. Cancer is another type of neoplasm marked by unlimited tissue growth. The essential feature that distinguishes cancer from nonmalignant neoplasms is that it does not stop growing.
Intestinal polyps are a common form of neoplasm. All intestinal polyps arise from the inner lining of the intestinal wall. This layer of mucosal tissue does the work of digestion. About 30% of the general population will develop intestinal polyps at some point in life, with the likelihood increasing with age. Most of these polyps are never noticed during a person's lifetime because they cause no problems. They are often discovered accidentally at autopsy. The primary importance of intestinal polyps is that 1% of them become cancerous. Because the polyps that eventually turn malignant cannot be identified in advance, they are all suspect.
Location of intestinal polyps
The chances of a polyp's becoming cancerous depend to some extent on its location within the digestive tract.
COLON. Ninety-five percent of all intestinal polyps develop inside the large bowel. There are several hereditary diseases that produce large numbers of intestinal polyps. These disorders include:
- familial polyposis of the colon
- Gardner's syndrome
- Lynch's syndrome
- Turcot's syndrome
- Peutz-Jeghers syndrome
- juvenile polyposis
All of these disorders are inherited in what is called an autosomal dominant pattern. This pattern means that the disorders are not sex-linked and that a child can inherit the disorder from either parent. In all of these hereditary disorders, the intestinal polyps appear during or after puberty. The first four diseases on the list have such a high rate of cancer of the large bowel (colon)—virtually 100% by the age of 40—that persons diagnosed with any of them should have the colon removed surgically in early adulthood.
STOMACH. The stomach's lining is host to polyps of a similar appearance, but there is no agreement as to their potential for becoming stomach cancer.
SMALL INTESTINE. Polyps in the small bowel do not seem to have malignant potential. Instead they can produce obstruction in either of two ways. A large polyp can obstruct the bowel by its sheer size. Smaller polyps can be picked up by the rhythmic contractions (peristalsis) of the intestines and pull the part of the bowel to which they are attached into the adjoining section. The result is a telescoping of one section of bowel into another, called intussusception.
Causes and symptoms
Population studies of colon cancer suggest that diet plays an important role in the disease, and by implication in the formation of colon polyps. The most consistent interpretation of these data is that animal fats—though not vegetable fats—are the single most important dietary factor. Lack of fiber in the diet may also contribute to polyp formation. Other types of polyps are too rare to produce enough data for evaluation.
Most polyps cause no symptoms. Large ones eventually cause intestinal obstruction, which produces cramping abdominal pain with nausea and vomiting. As colon polyps evolve into cancers, they begin to produce symptoms that include bleeding and altered bowel habits.
Routine screening for bowel cancer is recommended for everyone over the age of 40. Screening may be as simple as testing the stool for blood or as elaborate as colonoscopy. Colonoscopy is a procedure in which the doctor threads an instrument called a colonoscope up through the entire large bowel. Most polyps are in the lower segment of the colon, called the sigmoid colon. These polyps can be seen with a shorter scope called a sigmoidoscope. X ray imaging can also used to look for polyps. For x rays, the colon is first filled with barium, which is a white substance that shows up as a shadowed area on the film. The colon can also be filled with barium and air, which is called a double contrast study.
Because polyps take about five years to turn into cancers, routine examinations are recommended every three years.
All polyps should be removed as preventive care. Most of them can be taken out through a colonoscope. Complications like obstruction and intussusception are surgical emergencies.
Patients with hereditary disorders associated with polyps must undergo total colectomy early in adult life. All children of parents with these disorders should be screened early in adulthood, because half of them will have the same disease. For the bulk of the population, increased dietary fiber and decreased animal fat are the best preventives known at present. For the occasional intestinal polyp that arises in spite of good dietary habits, routine screening should prevent it from becoming cancerous.
Levin, Bernard. "Neoplasms of the Large and Small Intestines." In Cecil Textbook of Medicine, ed. J. Claude Bennett and Fred Plum. Philadelphia: W. B. Saunders Co., 1996.
Mayer, Robert J. "Gastrointestinal Tract Cancer." In Harrison's Principles of Internal Medicine, ed. Anthony S. Fauci, et al. New York: McGraw-Hill, 1997.
Silverstein, Fred E. "Gastrointestinal Endoscopy." In Harrison's Principles of Internal Medicine, ed. Anthony S. Fauci, et al. New York: McGraw-Hill, 1997.
J. Ricker Polsdorfer, MD
Autosomal dominance—A pattern of heredity in which a trait is inherited without respect to sex and from either parent. The hereditary diseases associated with intestinal polyps are all autosomal dominant.
Colectomy—Surgical removal of the large bowel.
Intussusception—The slipping of one section of the intestine inside an adjoining section. Intussusception can be caused by small intestinal polyps.
Mucosal—Refers to tissues that produce mucus, such as the digestive, genital and urinary tracts.
Neoplasm—A new growth of abnormal tissue.
Peristalsis—The rhythmic contractions of muscular tubes like the intestines that carry the contents along the tube.
Sigmoid—The S-shaped curve of the large intestine where the colon joins the rectum.